>but I think it's much more worthwile to dive into the particulars of why it failed
If you’re going to deep dive, define your terms: what did it fail at?
At being the future? Kind of a straw man, even if some people did argue that.
As a document interchange format? Between RSS — including podcasts! — Microsoft Office, and LibreOffice it seems to have done reasonably well.
As a data interchange format? Undoubtedly JSON is now preferred for this, but keep in mind that for several crucial years, as the “X” in “AJAX,” it paved the way for modern web apps via Google Maps, Gmail, and before that Outlook for the Web (literally the original AJAX app).
I think it’s not really interesting to examine how and where JSON displaced XML given this is not hard to grasp and well covered. Much more interesting to extract lessons that also work on JSON and predict what might displace it and where and how. For example it’s interesting that JSON is simpler than XML. Might it have discarded something valuable? The “X” is for extensibility - if we could extend JSON, which we can’t, how would we? First class date times might nice, mmm? Or to be able to add arbitrary other self describing data types? UUIDs?
What is XML still ideal for? Etc